Both anatomical and electrophysiological experiments are planned to elucidate the rostral hypothalamic site responsible for the proestrus surge of prolactin. It is established that the steroid estrogenis responsible for this surge, but its site and mode of action are not known. Recent evidence suggests that the suprachiasmatic nucleus (SCN) is involved in the surge, but that an immediately adjacent structure, the medial preoptic nucleus (MPN) is the site of steroid feedback. Indeed, earlier experiments have demonstrated that estrogen can alter the electrical excitability of these neurons. Initial experiments are designed to locate, by lesioning techniques, the site responsible for the surge in the freely-moving rat (SCN versus MPN). Secondly, the electrical activity of the SCN and the MPN will be studied inthe freely-moving rat and correlation established between the electrical activity and peripheral prolactin levels. Then, the in vitro slice preparation will be utilized to look at the cellular electrophysiology of the area responsible for the surge. Intracellular recording will be done in order to look at synaptic input, estrogen-sensitivity and chemo-sensitivity to the putative transmitters, serotonin and B-endorphin. Finally, intracellular staining with procion yellow will be done in conjunction with immunocytochemistry to identify the morphology and the cytochemistry of these neurons.